In a radio communication system, a base station apparatus and a radio communication terminal apparatus in the same network share the same frequency channel. When a plurality of networks are present in neighboring areas, inter-network interference may be avoided by causing the networks to use different frequency channels.
When the base station apparatuses are immobile, the inter-network interference may be avoided by designing the networks in advance so that the frequency channels to be used by the networks are different from each other. However, when the base station apparatuses are mobile, to previously expect which networks will approach each other is difficult and thus, to previously avoid the inter-network interference by the network design is difficult.
FIGS. 1A and 1B are explanatory views for an example of a scene in which inter-network interference occurs. Each of references 1, 100, and 200 denotes a network that includes a base station apparatus and radio communication terminal apparatuses. As illustrated in FIG. 1A, a case in which the networks 1 and 200 are adjacent and the networks 1 and 100 are not adjacent at some point in time is assumed. In this case, the inter-network interference may be temporarily avoided when the network design is performed so that the networks 1 and 200 use different frequency channels and the networks 1 and 100 use the same frequency channel.
However, as illustrated in FIG. 1B, when the network 100 approaches the network 1 after that, interference occurs between the networks 1 and 100 that use the same frequency channel.
As a technique for avoiding inter-network interference, for example, channel hopping described in Institute of Electrical and Electronics Engineers (IEEE) 802.15.6 is known. Even when interference is caused, the possibility of avoiding the interference at a next hopping timing is raised by causing the channel hopping to work continuously.
However, when the number of usable channels is small, the channels selected at a hopping timing may correspond coincidentally and the possibility of causing another interference is raised. Thus, there is also a method in which channels are changed only when interference is caused. In the description below, this technique may be referred to as “Related Art 1”.
As a relevant technique, an apparatus and a method for avoiding a channel collision in regional radio networks are known. A medium access controller that performs switching for a base station from a first channel to a second channel at time t is provided. The medium access controller includes a switching time delay circuit that delays the switching with regard to the time t until random delay time. This technique may be referred to as “Related Art 2”.
For example, see Japanese National Publication of International Patent Application No. 2009-523360.
In Related Art 1, recollision occurs when the frequency channels respectively reselected for the networks that interfere with each other are the same. FIG. 2 is an explanatory view for an example of a scene in which the recollision occurs. It is assumed that in an ith super frame, both of the networks 1 and 100 use a frequency channel Ch1 and a channel collision has occurred between the networks 1 and 100.
After that, in a (i+1)th super frame, the networks 1 and 100 switch the frequency channel for use so as to avoid the interference. However, since both of the networks 1 and 100 switch the frequency channels for use to a frequency channel Ch2, which is the same selection again, the channel collision occurs again.
After that, when the networks 1 and 100 switch the frequency channels for use to a frequency channel Ch3 and the frequency channel Ch1, respectively, in a (i+2)th super frame, the interference between the networks 1 and 100 is avoided.
In Related Art 2, since a delay time before channel switching is determined randomly, recollisions caused by switching the previous channel to the same channel at the same time as another base station may be reduced. However, still in Related Art 2, the possibility of switching the channel with the same delay time as the delay time for the other base station remains and in this case, the channel collision may occur again.